1. Field of the Invention
The invention relates in general to apparatus for making electrical measurements, and more specifically to apparatus for measuring the output voltage of cascade connected transformers.
2. Description of the Prior Art
Transformers are commonly connected in a cascade arrangement, in order to develop the high voltages necessary to test certain types of electrical apparatus, such as large condenser bushings for electrical power apparatus. Since the object to be tested is usually capacitive in nature, the actual ratio of the cascade arrangement is not the same as the nominal ratio, with the actual ratio being extremely sensitive to load capacitance. An IEEE paper by D. Train and P. Vohl entitled "Determination of Ratio Characteristics of Cascade Connected Transformers," IEEE Transactions on Power Apparatus and Systems, Vol. PAS-95, No. 6, November/December 1976, pages 1911-1918, describes in detail the problems involved in determining the actual ratio of cascade connected transformers, and it also discusses various techniques for measuring the voltage across the final transformer of the cascade arrangement. For example, a voltage divider arrangement may be used. Such arrangement, however, is costly. Instruments may be directly connected to the units of the cascade connection. This, however, requires that they be read by a telescope, or it requires that the intelligence be transmitted from EHV to ground potential via costly and sophisticated techniques, such as electronic, optical or acoustical techniques. The paper further describes previously known graphical techniques for ratio determination, which are based upon the measurement of the the admittance of the first unit of the cascade connection. A ratio versus input admittance curve is plotted by experimentally developing points on the curve. This is practical, because the overall ratio of output voltage to input voltage for cascade connected transformers increases linearly with an increase in input admittance, for capacitive loads. While the graphical method has many advantages over other prior art methods, it would be desirable to be able to directly indicate the voltage across the final unit of the cascade connection, if this can be accomplished without offsetting disadvantages, such as the connection of a voltage divider across the final unit, the use of telescopes, or costly, sophisticated arrangements for transmitting intelligence from a high potential to ground potential.